I get the feeling that this is solely an Alienware (and maybe just an Alienware laptop) issue but I'll give it a shot anyway because there are some very knowledgeable folk on these forums :

In the Display settings for Far Cry 2 there are three sliders for brightness, contrast and gamma. I can move the sliders no problem but when I return to the game (after clicking on Apply) I always find it has made absolutely zero difference to the appearance of the game, even if all three sliders are set to minimum or maximum. I am using Windows 7 64-bit and have the game running in DX10 with all graphics options maxed out. My GPU is a GeForce GTX 680m. I've used Google to try to find a solution but have found only one other person who has experienced the same problem, and his card was a 780m and therefore his computer was also a laptop and I wouldn't be surprised if it was an Alienware as nobody else seems to have had this issue. The game still looks great in a kind of drably sunny way so it's not a game breaker, it's just annoying.

Try manually setting Brightness/Contrast/Gamma in the configuration file.
Enter Documents\My Games\Far Cry 2 and search for GamerProfile.xml. Open the file with an editor and look for Brightness, Contrast and GammaRamp settings. Try different combination, save and start the game. I am not sure about the max values you can put there, but you should consider that the default values should be "1" for them all and a decimal increase (i.e. "1.1", "1.2") should be sufficient in most cases.